In the manufacture of integrated circuits (ICs), several testing operations are performed to verify the functionality of the circuits during fabrication. One such operation occurs prior to packaging the ICs, also referred to as "die," while the ICs are in wafer form. Many manufacturers refer to this testing operation as "wafer probe." One of the primary purposes of wafer probe is to run diagnostic tests to identify die which are not functioning properly, so that only functional die undergo remaining manufacturing operations, such as packaging. By segregating the malfunctioning die from the good die early on, IC manufacturers reduce costs associated with unnecessary, subsequent processing of units which will eventually be discarded. To further segregate defective die, some manufacturers use a hot temperature wafer probe operation. Hot temperature probing serves the same purposes as conventional probing, but additionally identifies die which malfunction at elevated temperatures, similar to temperatures the die would be exposed to during operating lifetime. The fallout of malfunctioning die usually increases if hot temperature probing is employed because die which marginally pass conventional wafer probe often fail hot temperature probe testing.
A hot temperature wafer probe operation involves placing a wafer having a plurality of ICs, or die, onto a chuck, which is quite similar to conventional wafer probing. However, in hot temperature wafer probe, the chuck is typically heated to a temperature much higher than room temperature, for example 100.degree. C. Heating the chuck thereby heats the wafer and each die on the wafer. Upon reaching the desired temperature, diagnostic testing is performed to identify malfunctioning die. Testing is accomplished by bringing a probe card having a plurality of probe leads, or probe needles, in electrical contact with an individual die. Electrical signals are sent through the probe card to the die via the probe leads, and responses to these signals are fed to a computer which determines if the die is functioning properly. As stated earlier, by elevating the temperature of the die during wafer probe, marginally functional die will fail diagnostic testing with an increased probability over room temperature testing. Thus, costs are further reduced over conventional wafer probe due to a reduced number of die undergoing subsequent processing. In addition to reducing costs, hot temperature probing also helps to eliminate the number of marginally functional die that might be sold to a customer.
Although there is a trend by IC manufacturers to implement hot temperature wafer probing, IC manufacturers have discovered problems associated with this type of testing. A significant problem is that the elevated temperatures of the chuck and the wafer tend to warp the probe card and change the position of the probe leads such that the leads do not align properly to contact points, or bonding pads, on the die. Probe lead misalignment can also be the result of probe lead movement during testing, in other words, the probe leads were initially aligned properly but moved during a point in the testing sequence. Probe lead movement, or drift, is caused by a temperature difference between the heated chuck and the probe card. The probe card is typically held at room temperature, or about 25.degree. C., and the chuck and wafer are held at a much higher temperature, near 100.degree. C. When the probe card is brought into close proximity to the chuck and the wafer, the temperature gradient will result in a temperature increase in the probe card and in the probe leads. The temperature change in the probe card, in turn, causes the card to warp and often moves the probe leads. Even if the temperature change is not sufficient to significantly warp the probe card, the probe leads may still be affected due to a softening of an epoxy material or other adhesive material which is used to hold the probe leads in place. The softening of the epoxy material allows the probe leads to shift positions, rather than remain in positions predefined by probe card manufacturers to meet a particular bonding pad configuration of an IC.
A few solutions to the problem of hot temperature probing have been proposed. One solution is the addition of a "broad stiffener" which acts to stiffen the probe card to prevent it from warping. However, as mentioned earlier, probe leads may shift position even though the probe card does not warp. Another solution has been to add a heat sink to the probe card to remove any heat introduced by the heated chuck and wafer. Yet, a heat sink does not eliminate the presence of a temperature gradient between the chuck and the probe card, which is the most prominent contributor to unwanted movement of probe leads. Still another solution is to make the probe card, and other elements of a probe card apparatus, from materials having low coefficients of thermal expansion. In doing so, the amount of warpage and overall movement of the probe leads is believed to be reduced in the presence of a temperature gradient. This solution has worked to an extent, however has not completely eliminated the problem of probe lead movement during wafer probe testing.
A frequently used approach to resolving the problem of probe lead movement during hot temperature probing is to bring the probe card and probe leads in close proximity to the heated chuck without actually making contact to a die. The temperature of the probe card and the probe leads will rise and will eventually stabilize at a temperature near the temperature of the chuck. Upon achieving a stabilized temperature, the probe leads reach a stabilized position such that the probe leads can be accurately aligned to the bonding pads on a die. Although the temperature of the probe leads will further increase upon contacting bonding pads on the die, the temperature change will be much less than if the probe card was initially at room temperature. Thus by "preheating" the probe card, the movement of the probe leads is minimized and the probe leads can be accurately aligned to bonding pads of a die. A significant disadvantage in using this approach in a manufacturing environment is that "preheating" the probe card for each wafer takes a considerable amount of time, on the order of 1-2 minutes per wafer. Inefficient use of time in "preheating" a probe card is a considerable disadvantage in a competitive industry in which fast cycle time is a key to a successful business. Therefore, a need exists for an improved probe card apparatus, and more specifically for an improved probe card apparatus for functional testing at elevated temperatures and a process for using the same, which significantly reduces probe lead movement without compromising cycle time.